Tracking system for laser surgery

ABSTRACT

Systems and methods are disclosed for aligning and confining laser beam exposure to a defined target region of biological tissue during laser surgery by employing a floating lens which is mechanically coupled to a reference element, such as a target tissue securing assembly, in order to compensate for movements of the target during surgical procedures. The floating lens forms part of the imaging optics and, by its design, direct the laser beam to follow the movements of the target tissue regardless of translational movements of the target.

BACKGROUND OF THE INVENTION

The technical field of this invention is laser surgery in which a laseris used to ablate biological tissue or otherwise treat regions of thebody by irradiation and, in particular, is directed to systems andmethods for precisely aligning and confining laser beam exposure to adefined target region during such surgery.

It is known to employ laser sources to erode, ablate, coagulate, alteror otherwise treat surfaces of biological materials. Such laserapparatus is in general relatively complex and demands highly skilleduse. For example, laser ablative techniques have been proposed to modifythe shape of sensitive surfaces, such as the cornea of the eye tocorrect vision defects. Extreme care must be taken to confine theablative procedures to the upper layers of the cornea and to avoiddamage to the basement membrane and the posterior endothelial lining ofthe cornea in such operations.

The use of a laser beam as a surgical tool for cutting incisions, aso-called laser scalpel, has been known for some time (see, for example,U.S. Pat. No. 3,769,963 issued to Goldman et al.). Lasers have also beenemployed for removal of skin pigmentation abnormalities, "birthmarks,"scars, tattoos and the like. Furthermore, lasers have been used forphotocoagulation of blood vessels, fusion of biological tissue andselective ablation of delicate biological structures, including thereprofiling or reshaping of the cornea of the eye to correct refractiveerrors in vision.

A technique for corneal reshaping, involving the use of a laserphotoablation apparatus, is known in which the size of the area on thesurface to which the pulses of laser energy are applied is varied tocontrol the reprofiling operation. In one preferred embodiment, abeam-shaping shaping stop or window is moved axially along the beam toincrease or decrease the region of cornea on which the laser radiationis incident. By progressively varying the size of the exposed region, adesired photoablation profile is established in the surface. For furtherdetails on this technique, see also Marshall et al., "Photo-ablativeReprofiling of the Cornea Using an Excimer Laser: PhotorefractiveKeratectomy," Vol. 1, Lasers in Ophthalmology, pp. 21-48 (1986), andU.S. Pat. No. 4,941,093 issued to Marshall et al., both of which areherein incorporated by reference.

Another approach involves the use of a graded intensity orphotodecomposable mask which varies the laser transmission to the targetsurface, thereby inducing variable ablative depths on the surface. Forexample, U.S. Pat. No. 4,856,513 entitled "Laser Reprofiling Systems AndMethods" which describes methodology for selectively eroding the corneathrough the use of an erodable mask. The mask absorbs the surface laserradiation in varying amounts across the corneal surface to provide thedesired surface profiles.

One problem of particular noteworthiness in laser corneal surgery andthe like is the need for precise alignment of the laser and the targetregion. Even slight movements of the target can create problems insofaras the reprofiling operations are typically dependent upon thecumulative effects of a number of precisely aligned, discrete ablationsteps. Moreover, in some procedures, the problem resides not only inprecise positioning of the laser with respect to the eye or othertarget, but also in precise positioning of intermediate opticalcomponents, such as, for example, alignment and angular orientation of abeam-shaping mask or aperture. While gross eye movements can beprevented through the use of a eye restraining cup or the like, theproblem of minor movements remains.

Various techniques have been described for tracking eye movements.However, these techniques are usually based on computer tracking ormodeling of the eye, coupled with pattern recognition algorithms whichattempt to detect, and/or compensate for, eye movements in real time.Such approaches have proved difficult to implement. Even when the eyemovements can be monitored in real time, the hardware necessary to steera laser beam in synchrony with such movements is, likewise,technologically complex and proned to errors. There exist a need forbetter techniques for tracking eye movements and for precisely aligningand confining laser beam exposure to the target region of the eye duringlaser surgery.

It is, therefore, an object of the present invention to address theproblem of eye tracking, such that compensation can be provided forslight involuntary or inadvertent motions during ophthalmic surgery.More generally, it is an object of the invention to provide better andmore reliable tracking mechanisms for laser surgical systems of varioustypes whenever precise alignment with a target is necessary ordesirable.

SUMMARY OF THE INVENTION

Systems and methods are disclosed for aligning and confining laser beamexposure to a defined target region of biological tissue during lasersurgery by employing a floating lens which is mechanically coupled tothe target, in order to compensate for movements of the target duringsurgical procedures. The floating lens forms part of the imaging optics,and by its design, directs the laser beam to follow the movements of thetarget tissue.

In one embodiment, a system is disclosed for tracking eye movementsduring laser ophthalmic surgery including an eye securing element (suchas an eye cup) for securing an eye during surgery and an opticalsubsystem for projecting ablative radiation onto a defined target regionof the eye. The optical subsystem further includes at least one fixedoptical element which is fixed in position during surgery, and at leastone floating optical element which is mechanical coupled to the eyesecuring element for movement therewith. The fixed optical element andthe floating element form an imaging system in which movements of eveare tracked by the floating optical element, such that the ablativeradiation remains imaged upon the target region.

The laser beam delivery system can employ two subsystems. The firstsubsystem can have at least one optically-active component, such as alens or mirror, which is located in a fixed position in reference to thelaser beam (e.g., in reference to the main body of the entire lasersurgical system). The second subsystem can be a floating one havingseveral degrees of freedom in reference to laser beam or the main bodyof the laser surgical system. The two subsystems form an imaging system,imaging an object located in the object plane, (e.g., an iris, mask, orstop) onto an image plane in such a manner that the changes in theposition of the image spot follow or track any changes in the positionof the floating subsystem along its degrees of freedom. The floatingsubsystem has, in one preferred embodiment, a target reference member,such as an eye cup and handle for manual operation, rigidly attached toits floating structure.

The spatial position of the reference member can be controlled directlyby the clinician or the reference member and can be attached directly tothe tissue. The first approach allows the clinician to direct freely thelaser beam to a selected tissue location. The second approach constrainsthe laser beam instead to follow the movements of the attached tissue.

The invention will next be described in connection with certainillustrated embodiments; however, it should be clear that variouschanges, additions and subtractions can be made without departing fromthe spirit or scope of the invention. In particular, it should beappreciated that the mechanical linkages illustrated in the followingdrawings are only one of a number of ways that comply can be achievedbetween a target securing member and a floating lens. The linkages canbe linear, proportional or non-linear depending upon the application.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention may be obtained byreference to the drawings in which:

FIG. 1 is a diagrammatic illustration of a tracking system for laserophthalmic surgery in accordance with the invention;

FIG. 2 illustrates a light restricting element incorporating anadjustable iris for use in the system of FIG. 1;

FIG. 3A through 3D illustrate diagrammatically successive steps inreprofiling a cornea with the adjustable iris of FIG. 2 to correctmyopia;

FIGS. 4A and 4B illustrate diagrammatically an alternative lightrestricting mechanism employing an erodable mask for use in the systemof FIG. 1 to correct myopia;

FIG. 5 is a more detailed schematic front view of a tracking system forlaser ophthalmic surgery, in accordance with the invention illustratingtracking motion in the Z direction;

FIG. 6 is a similar schematic front view of the system of FIG. 5illustrating tracking motion in the X direction;

FIG. 7 is a side view of the system of FIGS. 5 and 6 illustratingtracking in the Y direction; and

FIG. 8 is a diagrammatic illustration of a binocular, surgicalmicroscope for use in connection with tracking systems according to theinvention.

DETAILED DESCRIPTION

FIG. 1 illustrates system 10 for aligning and confining a laser beam 18to a defined target region 42, such as an eye during laser surgery. Thesystem 10 can include a laser source 12 for delivering ablative laserradiation, a light restricting element 14A and/or 14B for varying theexposure area over time, and an eye cup or other target reference member16 for coupling the target to a tracking system 20. (It should beappreciated that the light restricting element 14A/14B may not be neededin all embodiments; for example, if the laser beam 18, itself, has asuitable intensity profile, a selective reprofiling procedure can becarried out without modifying the beam shape or size over time.)Optionally, the system can further include a handle 17 for manuallydirecting the laser beam 18 to a desired target region.

In accordance with the present invention, the tracking system includesan optical subsystem 25 for projecting the ablative radiation onto thedefined target region 42, including at least one fixed optical element22 and at least floating optical element 24 which is mechanicallycoupled to the eye cup 16 or other target reference member via linkage28. The fixed optical element 22 and the floating element 24 will bothtypically be lens elements and will together form an imaging system inwhich movements of the target are tracked by the floating element 24 sothat the ablative radiation remains imaged upon the target regiondespite minor translational motion.

In FIG. 1, laser 12 can be a pulsed laser source, and the target surface42 can be a cornea, optically aligned to the laser 12. The laser, forexample, can be an excimer laser, and one preferred laser is anArgon-Fluoride laser having an ultraviolet (UV) characteristic emissionwavelength of about 193 nanometers. Alternatively, other pulsed UVlasers having both shorter wavelengths down to about 157 nanometers(e.g., a Fluoride laser) and longer wavelengths up to about 300nanometers may be useful in particular applications. In otherembodiments, mid infrared (IR) laser sources such as Erbium:YAG lasers(emitting at about 2,940 nanometers) generating pulsed radiation atwavelengths strongly absorbed by water can also be employed to produceablative effects.

Suitable irradiation intensities vary depending on the wavelength of thelaser, and the nature of the irradiated object. For any given wavelengthof laser energy applied to any given material, there will typically be athreshold value of energy density below which significant erosion doesnot occur. Above the threshold density, there will be a range of energydensities over which increasing energy densities give increasing depthsof erosion until a saturation value is reached. For increases in energydensity above the saturation value, no significant increase in erosionoccurs.

The threshold value and the saturation value vary from wavelength towavelength of laser energy and from material to material of the surfaceto be eroded, in a manner which is not easily predictable. However, forany particular laser and any particular material, the values can befound readily by experiment.

For example, in the case of ablating either Bowman's membrane or thestromal portion of the cornea by energy of wavelength 193 nanometers(the wavelength obtained from an ArF Excimer laser), the threshold valueis about 50 mJ per cm² per pulse, and the saturation value is about 250mJ per cm² per pulse. Suitable energy densities at the corneal surfaceare 50 mJ per cm² to one J per cm² per pulse for a wavelength of 193nanometers.

The threshold value can vary very rapidly with wavelength, and at 157nanometers, which is the wavelength obtained from an F₂ laser, thethreshold is about 5 mJ per cm² per pulse. At this wavelength, suitableenergy densities at the corneal surface are 5 mJ per cm² to one J percm² per pulse.

Most preferably, the laser system is used to provide an energy densityat the surface to be eroded of slightly less than the saturation value.Thus, when eroding the cornea with a wavelength of 193 nanometers (underwhich conditions the saturation value is 250 mJ per cm² per pulse), itis preferable to provide to the cornea pulses of an energy density ofabout 90 to about 220 mJ per pulse. Typically, a single pulse will erodea depth in the range 0.1 to 1 micrometer of collagen from the cornea.

The pulse repetition rate for the laser may be chosen to meet the needsof each particular application. Normally, the rate will be between 1 and500 pulses per second, preferably between 1 and 100 pulses per second.When it is desired to vary the beam size, the laser pulses may bestopped while the aperture or other beam shaping mechanism is changed.Alternatively, the beam size may be varied while the pulses continue. Ifa measurement device is used to monitor the erosion progress and controlthe laser system automatically, the beam size may be varied continuouslyat a controlled rate without interrupting the pulses.

In FIG. 2, one embodiment of a light restricting means 14A for use inthe system of FIG. 1 is shown. In this embodiment, an adjustable iris 15is employed to vary the exposure area over time. The leaves 15A of theadjustable iris can be programmed to slowly open (or conversely, slowlyclose), such that central region of the cornea receives a greatercumulative dose of ablative radiation than the peripheral regions. Bycontrolling the number of pulses emitted for each setting of theaperture and controlling the aperture size, the actual profile of theeroded surface of the cornea can be very closely controlled. FIGS. 3A-3D are schematic illustrations of how the beamshaping element of FIG. 2can operate to decrease the curvature of the cornea by selectivelyablating tissue.

In FIG. 3A, the intact surface layers of the cornea 42 are showncomprising the epithelium 60, Bowman's membrane 62, and the upperportion of the stroma 64. In FIG. 3B, a large aperture is employed toablate all (or a substantial portion) of the epithelial layer 60 of thecornea 42 in a region of the optical zone so as to expose the surface ofBowman's membrane 60. A first ablation region of wide, cross-sectionalarea is then created in Bowman's membrane 60 as shown in FIG. 3C. Anarrower region of further ablation as shown in FIG. 3D is then createdby employing a smaller aperture. The net effect is to create a flattenedcurvature. It should be clear that the actual procedure would be carriedout with substantially greater number of steps in order to achieve asmooth curve and minimize the step-affects. In some applications, it ispreferable to use an "opening iris" rather than a "closing iris," asillustrated in FIGS. 3B-3D; in this approach, the aperture is first setwith a small opening and then progressively opened larger. The netresult is the same: a general flattening of the corneal surface. Uponcompletion of the laser surgery, the epithelium regrows with a uniformthickness and produces a new corneal curvature determined by thereprofiling of the Bowman's membrane tissue. In certain applications, itmay be preferable to employ a wider optical zone and also ablate withpenetration into the stroma 64.

In FIGS. 4A and 4B, an alternative embodiment of the beamshaping means14B of FIG. 1 is shown in more detail. As illustrated, the beamshapingmeans 14B includes a mask element 13 incorporated into an eye cup orsimilar eye secure means 16. As illustrated, the eye cup 16 provides asupport structure having substantially rigid walls and a horizontalsurface upon which the mask is disposed. In the illustrated embodiment,the masking means 13 is an erodable mask, and it is disposed upon atransparent stage 66.

The entire structure can be placed upon the sclera of an eye, leavingthe corneal surface unobstructed. A flexible tube 70 can either supplyvacuum suction to the cup so as to clamp it to the eye or provide a flowof gas for removal of ablation residue. For further details on thestructure and composition of erodable masks, see U.S. Pat. Nos.4,856.513 and 4,994,058, herein incorporated by reference.

FIG. 4B illustrates the principle involved in eroding a surface toeffect reprofiling with a mask element. In FIGS. 4A and 4B, the surfacelayers of the cornea 42 are again shown, including the epithelium 60,Bowman's membrane 62, and the upper portion of the stroma 64. The mask13 is uniformly irradiated with a beam of radiation 18 obtained from thelaser source shown in FIG. 1.

During irradiation, the mask 13 is gradually ablated, and an increasingarea of the cornea becomes exposed to laser ablation. At the moment whenthe mask 13 has been wholly ablated, the surface of the cornea has beeneroded as indicated, to the extent necessary to complete the reprofilingover the area of the lens. As shown in FIGS. 4A-4B, the maximumthickness t₁ of the mask exceeds the minimum thickness t₂ by an amountequal to the maximum depth (d) of the corneal erosion desired. Bycontrolling the shape, thickness and/or composition of the mask 68,photoablation of the cornea can be precisely confined to either Bowman'smembrane 62 or the upper portions of the stroma 64. FIGS. 4A and 4Bagain illustrate a laser surgical techniques technique for correction ofmyopia. Similar lenses of appropriate shape can of course, be employedto remedy other forms of refractive errors, such as hyperopia,astigmatism and abnormal growths within the epithelium or the cornea.

In FIGS. 5-7, a mechanical linkage assembly 28 for coupling a targetsecuring element 16 and a floating optical element 24 is shown in moredetail. Typically, the target securing element 16 will inherentlyrestrain angular or rotational movements of the target vis-a-vis thelaser; this is particularly true with respect to cornea eyecup-typedevices which essentially prevent rotational movements of the eye. Thus,the movements, for which compensation must be provided, are constrainedto translational motions in the X,Y or Z directions (or combinationsthereof).

With reference first to FIG. 5, linkage assembly 28 can include a firstlinkage rod 72 or similar linkage means for tracking motion in theZ-direction. The linkage rod 72 serves to connect the eye cup 16 (orother target reference member) with a horizontal runner 86 which carriesthe floating optical element 24. The linkage rod 72 is joined to thelaser beam delivery tube 74 by one or more pivot arms, e.g., arms 76Aand 76B, as shown in FIG. 5. In the illustrated embodiment, the linkage72 is connected indirectly to the laser delivery assembly 74 by runners78A and 78B which permit the assembly 28 to move in lo channels 80A and80B along the Y-axis (with the assistance of bearings 82.) Nonetheless,the pivot arms 76A and 76B permit movement of linkage rod 72 up and downalong the Z-axis, such that movement of eyecup 16 results in acommensuration movement of floating lens 24.

In FIG. 6, linkage rod 72 is shown in phantom with further elements ofthe linkage assembly 28 superimposed on it in order to illustrate thetracking motion of the assembly with respect to movement of the eyecup16 along the X-axis. Linkage rod 72 includes a channel 84 (or similarguide) which permits runner 86 to move the left or right. Belt 88 andpulleys 90A, 90B, 90C and 90D cooperate to provide X-directionaltracking motion. As illustrated, belt 88 is fixed to flange 92 at bothof its ends by pins 94 and 96. Whenever eyecup 16 moves in theX-direction, belt 88 is pulled through the pulleys 90A-90D. Because belt88 is also fixed to runner 86 by pin 98, the movement of belt 88 aroundthe pulleys, also causes movement of the runner 86 which carries with itfloating lens 24.

A similar belt mechanism tracks movements in the Y-direction. FIG. 7 isa side view of the linkage assembly 28 illustrated in FIGS. 5 and 6,showing Y-direction motion. In FIG. 7, belt 102 is fixed at its two endsto guide rail 100 by pins 108A and 108B. When eyecup 16 moves in theY-direction, belt 102 is pulled through pulleys 104A, 104B, 104C and104D, because belt 102 is also attached to rod 72, e.g., by pin 106.Thus, floating lens 24 (which is linked to rod 72 by runner 86) moveswith the eyecup 16 because belt 102 pulls the assembly along thechannels 80A and 80B in the beam delivery housing 74.

As shown in FIG. 8, the tracking system 20 of the present inventionoptionally can also be used in conjunction with a surgical microscope 30or the like for viewing an eye or other target region during the lasertreatment procedure. As shown in FIG. 8, a beam splitting element 26(e.g., a dichotic mirror or the like) is disposed between fixed opticalelement 22 and floating optical element 24, such that the target surfacecan be visualized by the clinician during the procedure. (The microscope30 can also include appropriate UV filter elements to ensure that theview is not exposed to reflected UV radiation.) A visible light source31, also aligned with the optical axis, can be used to illuminate thetarget region and further enhance viewing.

What is claimed is:
 1. A system for tracking target tissue movementsduring laser surgery comprising:a laser source for emitting laserradiation; target securing means for securing target tissue regionduring surgery; and optical means for projecting the laser radiationonto said target region, the optical means further comprising: at leastone fixed optical element which is fixed in position during surgery; anda floating optical element which is mechanically coupled to the targetsecuring means for movement therewith, whereby the fixed optical elementand the floating element from an imaging system in which movements oftarget tissue region are tracked by the floating optical element, suchthat the laser radiation remains imaged upon the target region.
 2. Thesystem of claim 1 wherein the target securing means further comprises aneye cup adapted to be mounted upon the sclera of an eye.
 3. The systemof claim 1 wherein the target securing means further comprises a lightrestricting means for varying an exposure area within the target regionover time.
 4. The system of claim 3 wherein the light restricting meansis a masking means.
 5. The system of claim 4 wherein the masking meansfurther comprises an erodable mask.
 6. The system of claim 1 wherein theoptical means further comprises a first fixed lens element and a secondfloating lens element which cooperate to define an image plane at thesurface of the target region.
 7. The system of claim 1 wherein thefloating optical element further comprises a translational stage whichprovides for tracking of movements of the target securing means by thefloating optical element in at least two orthogonal directions.
 8. Thesystem of claim 7 wherein the translational stage provides for trackingin three orthogonal directions.
 9. The system of claim 1 wherein theoptical means further comprises a beamsplitter disposed between thefixed and floating optical elements to permit viewing of the targetregion during surgical procedures.
 10. The system of claim 9 wherein thesystem further comprises a surgical microscope optically coupled to thebeam splitter for viewing the target region.
 11. The system of claim 10wherein the system further comprises a source of visible light opticallycoupled to the beam splitter for illuminating the target region.